Plate-fin heat exchangers have particular application in cryogenic plants that are used in natural gas processing and in air separation. Such heat exchangers are typically fabricated from brazed aluminum heat exchanger cores that are fully brazed and welded in a vacuum brazing oven. In a typical brazing operation, fins, parting sheets and end bars are stacked to form a core matrix. The core matrix is placed in the vacuum brazing oven where it is heated to the brazing temperature in a clean vacuum environment.
Small air separation plants, typically less than 400 tons per day oxygen, utilize a heat exchanger that can comprise a single core. However, for higher flows and heat exchange duty, the plate-fin heat exchanger is constructed from several of such cores that are connected in parallel or series.
Heat exchanger efficiency is limited by the fact that each heat exchanger must be formed from individually brazed cores, which are in turn constrained in maximum cross-sectional flow area because the brazing ovens are limited in size. Typically, such brazing ovens have a length of between about 8.5 meters and about 10.0 meters and a width of between about 1.3 and 2.0 meters. Consequently, the length, width and height of any plate-fin heat exchanger is limited by the size of the furnace.
Typically, inlets and outlets for the fluids to be subjected to heat exchange are positioned at opposite ends of the longest dimension, namely, the length. If the heat exchanger were to be fabricated that is more compact, fin density would have to increase in order to provide an effective heat exchange area. Fin density is defined as the number of individual fins extending from the top to the bottom of a flow passage per inch of flow width. Obviously using higher fin density will result in a higher heat transfer surface area per unit volume. The increase in surface area inevitably comes at the expense of more frictional pressure drop. Hence, increasing fin density would increase the pressure drop. In applications such as air separation, air is compressed and purified and thereafter, the air is cooled to near its dew point prior to introduction into a distillation column. The product and waste streams produced by the distillation column flow back through the heat exchanger to cool the incoming air. Obviously, if the fin density were increased and therefore the pressure drop, the air would have to be compressed to a much higher pressure to overcome the increase in pressure drop for both the incoming air stream and the product and waste streams. While the degree of increase in compression that is required to overcome such increased pressure drop for the incoming air stream is not particularly severe, the amount of increased compression required for the product and waste streams is at an undesirable high level.
In order to lessen the pressure drop for a particular heat exchanger, it is known to utilize inlets and outlets for the flow along the length dimension (i.e. the longest dimension of the plates from which the heat exchanger is formed) of the heat exchanger in order to take advantage of a higher cross-sectional flow area inherent in such design. For example, in French Patent Application 2844040, the incoming air, nitrogen and oxygen is subjected to indirect heat exchange in alternating layers in which inlets and outlets for these components are situated along the length dimension. However, at each inlet of a plate-fin heat exchanger, distribution fins must be provided to redistribute the flow across the length dimension of the heat exchanger. The problem with such redistribution points is that they each cause the flow to change direction and therefore incur a pressure drop for such reason alone. Furthermore, where two or more streams are to be exchanged with the incoming air, separate layers for the streams to be warmed must be alternated with layers for the air stream to be cooled. Typically, two streams to be warmed are alternated with a stream to be cooled. The order of the layers in such a heat exchanger adds complexity to the design and the costs of fabrication.
Furthermore, since the flow of each layer must be distributed many times along the length, it is difficult to connect such heat exchangers in series should scale-up become necessary. This is due to the fact that the flow must be redistributed in a downstream heat exchanger and such redistribution can lead to an unacceptable pressure drop.
As will become apparent, the present invention provides a plate-fin heat exchanger in which the layering is configured such that layers for a stream to be cooled alternate with a single layer designed to accommodate the streams to be warmed. Furthermore, as will become apparent, a heat exchanger design in accordance with the present invention is far easier to scale up with a series connection between heat exchangers than in prior art designs. These and other advantages of the present invention will be discussed in detail below.